1. Field of the Invention
The present invention relates, in general, to a phase shift mask and, more particularly, to a half-tone type phase shift mask capable of preventing light from penetrating undesired areas and thus capable of obtaining a photoresist pattern with a superior smooth profile. The present invention is also related to a method for fabricating the half-tone type phase shift mask.
2. Description of the Prior Art
To achieve the recent trend of high integration in semiconductor devices, it has been required that wires be reduced in size and in the distance therebetween, and that unit elements, such as transistors and capacitors, become smaller. As a result, the patterns formed become finer, while the number of steps involved in fabricating highly integrated semiconductors has in general increased.
In conducting a typical photolithography process for a semiconductor device, there is employed a mask comprising a transparent substrate, such as quartz, on which optically opaque patterns are formed by coating chrome or aluminum and etching it with ion beam. However, it is difficult to form a fine pattern smaller than light resolution by use of the typical mask. Indeed, it is virtually impossible to obtain fine patterns with a line width of 0.5 .mu.m or less by use of current photoresist and light exposing equipment.
Meanwhile, very highly integrated semiconductor devices, such as 64M or more DRAM, demand fine patterns of 0.4 .mu.m or less. Various efforts have been made to meet such demands. In an effort to develop highly integrated semiconductor devices, phase shift masks were invented. In fact, the ultrafine patterns are accomplished by phase shift masks.
A phase shift mask is comprised broadly of a quartz substrate, optically opaque patterns and a phase shift material layer. The phase shift material layer is formed along with the optically opaque patterns on the quartz substrate, playing a role of shifting a beams of light at an angle of 180.degree.. Such phase shift mask is designed to keep the amplitude of a beam of light illuminated on a wafer constant during light exposure processes and to minimize the exposure caused by interference between a light passing through the phase shift material layer and a light passing through a pattern adjacent to the phase shift material layer, thereby improving the resolution of the photoresist film pattern.
In order to improve the contrast ratio of a light illuminated on a photoresist film, the phase shift material with a refractive index of n is formed in the shift mask in such a thickness as to shift to an angle of 160.degree.to 200.degree. the phase of an incident light with a wavelength of 1. For example, where the incident light is G-line or I-line and the phase shift material is an oxide, nitride or spin-on-glass (hereinafter referred to as "SOG"), the thickness of the phase shift material ranges from 3,400 to 4,000 Angstrom. Such a phase shift mask allows patterns with a width of 0.3 .mu.m or less even when conventional photoresist films and light exposing equipments are employed.
A half-tone type phase shift mask is utilized, for form, inter alia, contact hole patterns, and its process capability has been proven to be better by 50% or more. In methods for the fabrication of semiconductor devices, the half-tone type phase shift mask is especially useful in forming fine contact holes of 64M or more scale.
Now, in order to better understand the background of the present invention, a conventional half-tone phase shift mask will be described in connection with FIGS. 1 and 2.
First, referring to FIG. 1, there is shown a cross section of a conventional phase shift mask. As shown in this figure, the conventional phase shift mask comprises a quartz substrate 11 on which a half-tone type light-penetrating film pattern 13 along with a phase shift film pattern 15 is formed so as to provide a window exposing the quartz substrate 11 therethrough. The half-tone type light-penetrating film pattern 13 is made of chrome or aluminum oxide and has such a thickness so that it is penetrated by about 5 to 20% of an incident light. The phase shift film pattern 15 is made of an oxide, nitride or SOG with a predetermined thickness.
When a light is illuminated on such half-tone type phase shift mask, the intensity of light is maximized at the central area of the window and reduced to zero at the boundary between the window and the pattern. The intensity of light at areas provided with no window, is, although very weak, detected in an amount of about 5 to 20% of that of the incident light. This is attributed to the fact that the light penetrates through the half-tone type light-penetrating film pattern 13. Of course, the intensity of the passing through light is proportional to that from a light source. In addition, the intensity of this light is not constant but different even at the areas in which a window is not provided.
Accordingly, in the case while a positive photoresist film 18 coated on a semiconductor substrate 17 is illuminated by use of a conventional half-tone type phase shift mask, the area of the positive photoresist film 18 which is intended to be patterned becomes illuminated because of the penetration of light through the light screen and thus becomes patterned after development, as shown in FIG. 2. As a result, there is obtained a photoresist film pattern having an uneven surface.
Such photoresist film pattern with the stepped surface is a factor which decreases production yield and reliability of a semiconductor device. For example, when this conventional photoresist film pattern is used as an etch mask or an ion implantation mask, a layer under the mask may be damaged or ions may be implanted into unnecessary areas.